Question: In chemistry class, the girl to boy ratio is $9$ to $10$. If there are $18$ girls, how many boys are there?
Solution: A ratio of $9$ girls to $10$ boys means that there is a group of $9$ girls for every group of $10$ boys. If there are $18$ girls, then there are $2$ groups of $9$ girls each. Because we know the ratio is $9:10$ , there must be $2$ groups of $10$ boys each. There is a total of $20$ boys in chemistry class.